Lab on a chip devices suffer from difficulties associated with sensing and transporting small samples. There is enormous potential of integrating optical and microfluidic elements into lab-on-a-chip devices, particularly in enhancing fluid and particle manipulations. Traditionally accomplished through direct particle manipulation with laser tweezers, or indirectly using optically induced microfluidic effects, the precision with which particles can be manipulated with these techniques makes them particularly useful for applications ranging from flow cytometry to self-assembly.
Fundamentally however, these free-space systems are limited in two ways. Firstly, diffraction limits how tightly the light can be focused and thereby the overall strength of the trap. Secondly, the trapping region has a very short focal depth preventing the continuous transport of nanoparticles via radiation pressure. To improve trapping stability a number of near-field methods have recently been developed. In one prior art method, interfering Gaussian beams are reflected off a prism surface to sort 350 nm polystyrene beads. In a further method, localized plasmonic resonances in surface bound metallic nanostructures are used to trap 200 nm dielectric particles.
Waveguide based optical transport is analogous to these near field methods in that the evanescent field extending into the surrounding liquid serves to attract particles to the waveguide. However, particles also experience photon scattering and absorption forces which propel them along it for a distance limited only by the losses in the system. Recent efforts in this area have demonstrated the sustained propulsion of dielectric microparticles, metallic nanoparticles and cells. The limitation which prevents these systems from manipulating smaller matter, including biomolecules, is that the particles only interact with the small portion of total transported light since the majority of it is confined within the solid core of the waveguide.